text: mTECs as APCs reveal some drawbacks on population level. Their numbers in thymic medulla reach only 100,000 per 2-week-old thymus. Furthermore, average lifespan of mTECs does not exceed 2–3 days, probably due to only known PGE activator Autoimmune regulator (Aire), which requires for its proper function generation of DNA double strand breaks. And last but not least, each TRA is expressed only by 1-3% of mTEC population. These facts decrease the chance of efficient recessive or dominant tolerance.